Passive micro or macro antennas, for example antennas used in mobile radio communications, comprise an antenna network with power splitters, passive amplitude tapers (attenuators) and passive phase shifters to feed multiple ones of antenna elements which form the antenna array. Each one of the individual antenna elements has a radiation pattern which is superposed and results in an overall radiation pattern of the antenna array in the far field. Typically, the antenna array will be arranged in a vertical manner (one column) and each of the antenna elements in the antenna array will be uniformly excited. The resulting vertical radiation pattern has a main lobe with a 3 dB half-power beam width and several sidelobes which are symmetrically arranged on both sides of the main lobe. In many situations, the several sidelobes are not an issue as long as the main lobe is pointing to the horizon and the goal of the antenna array is to maximise coverage. However, in cellular communication systems, it is necessary to have a limited coverage of the antenna array which corresponds to the size of a cell fed by the antenna array. Since cellular communication systems are limited by interference between adjacent ones of the cells, the goal of the antenna array in such cellular communication systems is to reduce as much as possible any interference from the antenna arrays arranged in adjacent ones of the cells. This reduction is implemented by the selection of correct frequencies and planning the cells based on topology data and wave tracing models. It is found in practice, that real propagation conditions are different from those which are predicted. For this reason, the antenna array can physically be “downtilted” so that the main lobe does not point at the horizon but towards the ground. The downtilting is done either by a mechanically driven or an electrical tilt mechanism. One disadvantage of the mechanical downtilting of the antenna array is that a first (upper) one of the sidelobes above the main lobe could point to the horizon and as a result cause unwanted interference with the adjacent ones of the cells. The consequence is that the fixed side lobe suppression of the antenna array needs to be designed in such a way that, for all of possible downtilt values, the worst case side lobe suppression is fulfilled. This is typically implemented by fixed amplitude tapering that results in a lower overall gain of the antenna array.
In the case of active antenna arrays which have transceivers attached to each one of a plurality of antenna elements, a flexible downtilting can be achieved by beam forming. The beam forming is implemented by multiplying individual complex values to each one of the individual transmission signals per antenna element. The advantage of beam forming through active antenna arrays compared to passive antenna arrays is that the downtilt is easily adjustable by digital signal processing instead of mechanically or by the electrical motors. In contrast to the mechanical downtilting, the physical phase shifting or digital beam forming affects the relationship between the main lobe and the sidelobes. This change in relationship can result in the transmission of unacceptable interference to adjacent ones of the cells in particular, if the beam pattern is tilted far down low. To avoid this one has to design the relation between effective radiated power in the main beam and the required sidelobe suppression independent on the tilt setting, i.e. it requires an inefficient worst case design.
A further issue which is known to occur in active antenna arrays is the failure of individual ones of the transceivers. The failure of the transceivers will not only result in an overall power degradation of 1/M (M being the total number of active elements) but also in a distortion of the radiation patterns. The distortion of the radiation pattern primarily results in the increase of the strength of the sidelobes which can also cause unwanted interference in adjacent ones of the cells.
A similar problem also occurs in horizontal or two-dimensional beam forming using multidimensional antenna arrays. If, for example, the beam forming is used in spatial-division multiple access (SDMA) techniques the goal of the antenna array is to point its power only to a particular point of interest and to produce low intracell interference outside of the main lobe.